1. Field of the Invention
The present invention relates to a communication apparatus, for example, a facsimile machine, which executes communication according to the protocols that are defined by Recommendations T.30, V.8 and V.34 of the ITU-T (International Telecommunication Union for Telecommunications).
2. Description of the Related Art
Recommendation T.30 of the ITU-T defines a protocol for transmitting text via facsimile over a public switched telephone network(PSTN), Recommendation V.8 specifies a session start protocol for data transmission over the PSTN, and Recommendation V.34 defines the specifications of a modem which operates at a data signal rate of up to 33600 bps that is used in the PSTN and a 2-wire, point-to-point telephone-type private line.
FIG. 13 is a diagram showing a protocol signal in a communication apparatus according to first prior art. Here, a calling station and a called station are both operable according to the protocols of Recommendations T.30, V.8 and V.34 of the ITU-T. After a line connection between the calling station and called station is established, the called station transmits a modulated answer tone signal(ANSam). Upon reception of the ANSam signal, the calling station transmits a calling menu signal(CM signal). When the called station receives the CM signal within a predetermined ANSam transmission time t1 defined by Recommendation V.8, the called station transmits a common menu signal(JM signal). A modulation mode between the calling station and the called station is determined by exchanging the CM signal and the JM signal. Upon reception of the JM signal, the calling station transmits a calling menu end signal(CJ signal), which is in turn received by the called station. After the calling station and called station complete the protocol of Recommendation V.8, the protocol of Recommendation V.34 is carried out.
FIG. 14 is a diagram showing a protocol signal in a communication apparatus according to second prior art. In this prior art, a calling station and a called station can also operate according to the protocols of Recommendations T.30, V.8 and V.34. After a line connection between the calling station and called station is established, the called station transmits an ANSam signal. When the calling station does not receive the ANSam signal and thus transmits no CM signal, the called station cannot receive the CM signal within the predetermined ANSam transmission time t1. At this time, the called station interrupts transmission of the ANSam signal and transmits a digital identification signal(DIS) in which bit 6 is set, i.e., bit 6=1. Upon reception of the DIS, the calling station transmits a calling indication signal(CI signal). When receiving the CI signal, the called station transmits the ANSam signal. After the calling station receives the ANSam signal, the protocols of Recommendations V.8 and V.34 are executed as illustrated in FIG. 13.
FIG. 15 is a diagram showing a protocol signal in a communication apparatus according to third prior art. Here, a called station can operate according to the protocols of Recommendations T.30, V.8 and V.34, while a calling station can operate according only to the protocol of Recommendation T.30. After a line connection between the calling station and called station is established, the called station transmits an ANSam signal. When the calling station does not receive the ANSam signal and thus transmits no CM signal, the called station cannot receive the CM signal within the predetermined ANSam transmission time t1. At this time, the called station interrupts transmission of the ANSam signal and transmits a DIS with bit 6 set. Upon reception of the DIS, the calling station transmits a digital command signal(DCS) or a non-standard facilities set-up signal(NSS), and the called station receives the DCS or NSS. In this manner, the calling station and the called station execute the protocol of Recommendation T. 30.
FIG. 16 is a diagram showing a protocol signal in a communication apparatus according to fourth prior art. Here, a called station is operable according only to the protocol of Recommendation T.30, while a calling station is operable according to the protocols of Recommendations T.30, V.8 and V.34. After a line connection between the calling station and called station is established, the called station transmits an answer tone signal(ANS signal). The calling station does not receive the ANS signal and does not therefore transmit a CM signal. The called station transmits the ANS signal for a predetermined ANS transmission time t2 defined by Recommendation V.25, and then transmits a DIS with bit 6 reset, i.e., a DIS with bit 6=0. The calling station, when receiving the DIS, transmits a DCS or NSS, and the called station receives the DCS or NSS. In this manner, the calling station and the called station execute the protocol of Recommendation T.30.
Some examples of such communication apparatuses are disclosed in, for example, Japanese Unexamined Patent Publications JP-A 8-79483 (1996), 7-298027 and 8-214132 (1996). In JP-A 8-79483, a communication apparatus operable according to the protocol of Recommendation V.8 is permitted to receive data both from a station which is operable according to the protocol of Recommendation V.8 and a station which is operable according to the protocol of Recommendation T.30 in the following manner. When receiving the CM signal within the ANSam transmission time t1, the apparatus transmits the JM signal to execute the protocol of Recommendation V.8. When the apparatus cannot receive the CM signal within the ANSam transmission time t1, on the other hand, the apparatus transmits an nonstandard facilities signal(NSF signal), a called subscriber identification signal(CSI signal) and DIS, and upon reception of the CM signal thereafter, the apparatus transmits the JM signal and then executes the protocol of Recommendation V.8, while upon reception of the NSS, transmitting subscriber identification signal(TSI signal) and DCS, the apparatus executes the protocol of Recommendation T.30.
In Japanese Unexamined Patent Publication JP-A 7-298027 (1995), in order to enable a subscriber terminal operative according to the protocol of Recommendation V.8 and a subscriber terminal inoperative according to the protocol of Recommendation V.8 to communicate with each other, a communication apparatus operative according to the protocol of Recommendation V.8 transmits a CI signal after transmission of a calling tone signal(CNG signal), and thereafter upon reception of an AI signal or ANSam signal, executes the protocol of Recommendation V.8 after transmission of the CM signal, whereas when receiving the NSF, CSI and DISs, the apparatus executes the protocol of Recommendation T.30 after transmission of the NSS, TSI and DCS. Further, in JP-A 8-214132 (1996), to effectively utilize Recommendation V.8, the communication apparatus transmits the CM signal upon reception of the ANSam signal from a called station, and transmits the CJ signal upon reception of the JM signal from the called station.
Japanese Unexamined Patent Publication JP-A 8-214136 (1996) discloses that when a transmitting subscriber terminal can not receive a JM signal from a called station during a predetermined period of transmission of a CM signal, the transmitting subscriber terminal executes switching among transmission modes so that the JM signal is received in the half duplex transmission mode after transmission of the CM signal and in the case where a CJ signal is transmitted after reception of a JM signal at a calling station a called subscriber terminal executes switching among transmission modes, when the CM and CJ signals can not be received during transmission of the JM signal, so that the transmission period of the JM signal is extended and the CJ signal from the calling subscriber terminal is received in the half duplex transmission mode.
FIG. 17 is a diagram depicting a protocol signal for the conventional communication apparatuses when a communication error occurs. When a calling station can not receive the ANSam signal which has been transmitted from a called station, the calling station does not transmit the CM signal and waits for the ANSam signal or DIS. When the called station erroneously acknowledges that it has received the CM signal, the called station transmits the JM signal. As the calling station is waiting for the ANSam signal or DIS, the calling station does not receive the JM signal and thus transmits no CJ signal. Consequently, the called station cannot receive the CJ signal and thus keeps transmission of the JM signal. When a predetermined signal transmission time T1 defined by Recommendation T.30 passes, the calling station recognizes it as a communication error and terminates the communication. When the called station does not receive the CJ signal within the signal transmission time T1, the called station interrupts transmission of the JM signal, determines that a communication error has occurred, and terminates the communication. This undesirably disables transmission and reception of desired data.
FIG. 18 is a diagram depicting a protocol signal for conventional communication apparatuses when another communication error occurs. Upon reception of the ANSam signal from a called station, a calling station transmits the CM signal. The calling station keeps transmission of the CM signal until the calling station receives the JM signal from the called station. When the calling station does not receive the JM signal then, the calling station determines it as a communication error, and terminates the communication.
This will be discussed specifically. In the case where the called station is inoperable according to the V.8 protocol, when the called station transmits the ANS signal and the calling station mistakes the ANS signal received for the ANSam signal, the calling station transmits the CM signal, as shown in FIG. 18. The CM signal is continuously transmitted until the calling station receives the JM signal. Meanwhile, however, the called station transmits the ANS signal for the ANS transmission time t2, and then transmits the DIS with the bit 6 reset. Thereafter, when the called station does not receive a signal (flag) of the channel 2 of the V.21 within a predetermined time T4, which is defined by Recommendation T.30, the called station transmits the DIS again, and repeats the transmission of the DIS within the signal transmission time T1 until the called station receives an effective signal of the V.21 channel 2. As the calling station is transmitting the CM signal, however, the signal transmission time T1 will elapse without the called station receiving the effective signal of the V.21 channel 2. Accordingly, the called station determines that a communication error has occurred, and terminates the communication. Undesirably, therefore, transmission and reception of desired data cannot be accomplished.